Those in the wide ranging materials joining industries have recognized the benefits of friction stir welding (FSW) since its invention, only to be precluded from widespread application due to a number of factors. One such long-recognized need has been that of providing a simple, reliable, and inexpensive joint clamping mechanism that provides manufacturing flexibility and overcomes the limitations of current clamping systems.
FSW is a relatively simple method of solid phase welding developed by The Welding Institute in the early 1990's. The process utilizes a specially shaped nonconsumable cylindrical tool with a profiled probe, often threaded, extending from a shoulder of the tool that is rotated and plunged into a joint formed by abutting edges of the workpieces that are to be joined until a surface of the shoulder contacts the surface of the workpieces. The rotating tool plasticizes a region of the workpieces around the probe and beneath the shoulder. The tool is then advanced along the joint. The rotation of the tool develops frictional heating of the workpieces and the tool forces plasticized workpiece material from the leading edge of the tool to the rear of the tool, while confining the plasticized material from above by the shoulder, where it consolidates and cools to form a high quality weld.
The FSW tool is generally formed as a cylindrical piece with a shoulder face that meets a probe that projects from the shoulder face at a right angle, as illustrated in U.S. Pat. Nos. 5,460,317 and 6,029,879. In some instances, the probe actually moves in a perpendicular direction in an aperture formed in the face of the shoulder, as illustrated in U.S. Pat. Nos. 5,611,469, 5,697,544, and 6,053,391. The face of the shoulder may be formed with an upward dome that is perpendicular to the probe, as illustrated in U.S. Pat. Nos. 5,611,479, 5,697,544, and 6,053,391. The dome region and an unobstructed shoulder face to probe interface are considered essential for the proper frictional heating of the workpiece material. The dome region serves to constrain plasticized material for consolidation at the trailing edge of the FSW tool so as to prevent it from extruding out from under the sides of the tool.
Since FSW is a solid-state process, meaning there is no melting of the materials, many of the problems associated with other fusion welding methods are avoided, including porosity, solidification cracking, shrinkage, and weld pool positioning and control. Additionally, FSW minimizes distortion and residual stresses. Further, since filler materials are not used in FSW, issues associated with chemical segregation are avoided. Still further, FSW has enabled the welding of a wide range of alloys that were previously unweldable. Yet another advantage of FSW is that it does not have many of the hazards associated with other welding means such as welding fumes, radiation, high voltage, liquid metals, or arcing. Additionally, FSW generally has only three process variables to control (rotation speed, travel speed, and pressure), whereas fusion welding often has at least twice the number of process variables (purge gas, voltage, amperage, wire feed speed, travel speed, shield gas, arc gap, just to name a few). Perhaps most importantly, the crushing, stirring, and forging of the plasticized material by the FSW tool produces a weld that is more reliable than conventional welds and maintains material properties more closely to those of the workpiece properties, often resulting in twice the fatigue resistance found in fusion welds.
Despite all the advantages of FSW it has only found very limited commercial application to date due to many difficulties associated therewith, including both the machine cost as well as the tooling cost. Machine cost refers to the cost of the actual FSW apparatus, whereas tooling costs refers to the actual tooling components as well as the clamps and related support structure, or backing framework. Perhaps the greatest difficulty to date has been associated with securely clamping the workpieces during the welding process. Typically, when the workpieces edges are abutted to create a joint, the workpieces must be rigidly clamped to a backing bar in a manner that prevents the joint from being forced apart as the probe is plunged into the joint. The tool is generally forced into the workpieces at loads exceeding 500 pounds of force, while rotating at hundreds of revolutions per minute. As a result, in even the simplest joining procedures elaborate clamping systems are used in which a plurality of clamps is installed over the entire length of each workpiece. Such clamping is extremely time consuming to set-up and the required hardware is expensive. As a result, FSW has been limited to welds of simple travel paths on relatively simple components thereby preventing widespread use of FSW, and particularly FSW on components having complex curvature.
Prior FSW systems have utilized a plurality of rollers rigidly secured to the FSW apparatus to limit the depth that the probe may enter the workpieces, as perhaps best illustrated in U.S. Pat. No. 5,971,247. The prior art rollers have generally been large and heavy, often having four or more rollers on each workpiece, and are located away from the weld area. Such rollers systems have generally only been practical in flat table FSW setups wherein flat workpiece sheets are clamped to a flat table and are then subjected to the FSW apparatus and clamping rollers.
The instant invention addresses many of the shortcomings of the prior art and allows for previously unavailable benefits. A method and apparatus for local clamping during the FSW process has long been needed. The system of the present invention is designed to overcome the clamping limitations of FSW. The system of the present invention does not introduce limitations into the FSW process and opens up the application of FSW to a wide variety of applications which were previously uneconomical. The method and apparatus utilize clamping means in the immediate vicinity of the FSW tool thereby reducing, if not eliminating, much of the traditional heavy clamping required in conventional FSW, and further improving access to the weld joint, improving weld quality, and significantly reducing manufacturing set-up time and tooling costs. The instant invention also can adapt to workpieces of complex curvature, incorporate the use of multi-axis computer control systems, as well as provide an adaptive load control system.